Abstract: The effects of the Nb content on the structure and magnetic properties of the nanocrystalline Fe_76.5-xCu₁Nb_xSi_13.5B₉ alloys have been studied. The grain size of the α-Fe(Si) decreases with increasing Nb content, x. Meanwhile, the volume fraction, the Si content and the degree of order of the DO₃ order zone in the α-Fe(Si) decrease. The Nb content in the nanocrystalline Fe_76.5-xCu₁Nb_xSi_13.5B₉ alloy has an effect on the structure of the residual amorphous phase. The magnetostriction of the alloys increases with increasing x. The maximal initial permeability and the minimal coercivity are obtained with x=3.